Solid Mechanics

The equations of motion for linear vibrating systems are well known and widely used in both mechanical and electrical devices. However, when students are introduced to these, they are frequently presented with solutions which are either essentially underived or inadequately so.

This brief presentation will attempt to address this deficiency and hopefully show the derivation of the equation of motion for an undamped oscillating system in a more rigourous way.

Consider a simple spring/mass system without a forcing function. The equation of motion can be expressed as

where x(t) is displacement as a function of time, m is the mass of the system, and k is the spring constant. The negative sign on the right hand side of the equation is not an accident, as the spring force always opposes the motion of the mass, and is the result of using a mechanical engineers’ “free body diagram” method to develop the equation.

Solutions to this equation generally run in two forms. The first is a sum of sines and cosines:

But it’s more common to see it in the form of

The latter is simpler and easier to apply; however, it is seldom derived as much as assumed. So how can it be obtained from the original equation?

Let us begin by considering the original differential equation. With its constant coefficients, the most straightforward solution would be a solution where the derivative (and we, of course, would derive it twice) would be itself. This is the case where the function is exponential, so let us assume the equation to be in the form of

(I had an interesting fluid mechanics/heat transfer teacher who would say about this step that “you just write the answer down,” which we as his students found exasperating, but this method minimises that.)

Substituting this into the original equation of motion and diving out the identical exponentials yields

Solving for α yields

The right hand term is the natural frequency of the system, more generally expressed as a real number:

Thus for simplicity the solution can be written as

At this point it is not clear which of these two solutions is correct, so let us write the general solution as

Because of the complex exponential definition of sines and cosines, we see the beginning of a solution in simply one or the other, but at this point the coefficients are in the way.

These coefficients are determined from the initial conditions. Let us consider these at t=0:

Substituting these into our assumed general solution yields

The coefficients then solve to

It is noteworthy that the two coefficients are complex conjugates of each other.

Since the general solution is written in exponential form, it makes sense that, if the coefficients are to be removed so we can enable a direct solution to a sine or cosine, they too should be in exponential form. Converting the two coefficients to polar form yields

Substituting these coefficients into the general solution, we have

Factoring out the radical and recognising that the arctangent is an odd function,

The quantity in brackets is the complex exponential definition of the cosine, since the two exponents are negatives of each other. The solution can thus be written as

If we define

the solution is

which can be rewritten in a number of ways.

If the dampening is added, the problem can be solved in the same way, but the algebra is a little more complicated, and we will end up additionally with a real exponential (decay) in the final solution.

This derivation demonstrates the power of complex analysis as applied to differential equations even in a simple way.

More examples of this kind of thing are here and here.

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He’s not widely known outside of the fields he specialised in, but Adhémar Jean Claude Barré de Saint-Venant (1797-1886, usually known in the Anglophone world as simply Saint-Venant) was one of the premier scientists, engineers and mathematicians of the nineteenth century. His accomplishments were many and include the following:

Successful derivation of the Navier-Stokes Equations for a viscous flow before Stokes; these equations are the basis of computational fluid dynamics and the analysis of things that fly.
Systematisation and development of methods in the theory of elasticity of solids, including his semi-inverse methods for torsion, important in things such as automobile crank shafts.
Methods for the analysis of wave mechanics in bars, which we see in many places, from musical instruments to driven foundation piles.

Saint-Venant was born into a royalist, aristocratic, traditionally Roman Catholic family at a time when it was not safe to be any of these: the French Revolution, at that point stumbling from the Reign of Terror to control of France–and most of Europe–by Napoleon Bonaparte. It was about the latter where Saint-Venant made a statement about himself that got him into trouble with the “new” Europe. As described in S. Timoshenko’s History of Strength of Materials:

The political events of 1814 had a great effect on Saint-Venant’s career. In March of that year, the armies of the allies were approaching Paris and the students of the École Polytechnique were mobilized. On March 30, 1814, they were moving their guns to the Paris fortification when Saint-Venant, who was the first sargeant of the detachment, stepped out from the ranks with the exclamation: “My conscience forbids me to fight for an usurper…” His schoolmates resented that action very much and Saint-Venant was proclaimed a deserter and never allowed to resume his study at the École Polytechnique.

Saint-Venant’s statement of conscience was at once a political and religious statement, and “progressives” of his day didn’t miss either. The French, then and now innocent of anti-discrimination legislation or sentiments, made his life miserable. The École Polytechnique was and is France’s premier technical institute of higher learning; getting kicked out of it was the equivalent of, say, being expelled from Princeton or MIT. He worked in the powder industry for nine years, then was admitted to the École des Ponts et Chausées, where his fellow students shunned him. He graduated first in his class anyway and began his illustrious career in technical things both theoretical and practical.

In spite of his difficulties within France, his reputation outside of her was another matter. When François Napoleon Moigno wrote his book on statics, he discovered the following:

He (Moigno) wanted the portion on the statics of elastic bodies to be written by an expert in the theory of elasticity, but every time he asked for the collaboration of an English or a German scientist, he was given the same answer: “You have there, close to you, the authority par excellence, M. de Saint-Venant, consult him, listen to him, follow him.” One of them, M. Ettingshausen, added: “Your Academy of Sciences makes a mistake, a great mistake when it does not open its doors to a mathematician who is so highly placed in the opinion of the most competent judges.” In conclusion Moigno observes: “Fatally belittled in France of which he is the purest mathematical glory, M. de Saint-Venant enjoys a reputation in foreign countries which we dare to call grandiose.”

The French finally broke down and admitted Saint-Venant into the Academy of Sciences in 1868. He continued his work, much of it from his home, up until the time of his death. When the President of the Academy announced that passing, he made the following statement:

Old age was kind to our great colleague. He died, advanced in years, without infirmities, occupied up to the last hour with problems which were dear to him and supported in the great passage by the hopes which had supported Pascal and Newton.

Europeans of the time would not have missed the import of the last statement: Pascal and Newton were Christians, and Saint-Venant was being identified with them as one also. It was also a statement that Saint-Venant, for all of his achievements and interests which have enriched the world, also had an eternal goal as well.

There’s no evidence that Saint-Venant was ostentatious in his faith walk; descriptions of his life show the contrary. And–shock to today’s atheist–there’s no evidence that it ever impeded the progress of his research or his thought. As the statistician and eugenicist Karl Pearson, no friend of Christianity, noted:

The more I studied Saint-Venant’s work, the more new directions it seemed to me to open up for original investigation of the most valuable kind. It suggested innumerable unsolved problems in atomic physics, in impact, in plasticity and in a variety of other branches of elasticity, which do not seem beyond solution, and the solution of which if obtained would be of extreme importance. I felt convinced that a study of Saint-Venant’s researches would be a most valuable directive to the several young scientists, whose recent memoirs shew their interest in elasticity as well as their mathematical capacity. Many of the problems raised by Saint-Venant’s suggestive memoirs were quite beyond my powers of analysis, and I recognised that the most useful task I could undertake, was by a careful account of the memoirs themselves to lead the more competent on to their solution.

saint-venant-wave-equation

The biggest impediment he had to face was the blowback from his stand at the École Polytechnique, and that came from his secularist colleagues. But, when the end came, all of his colleagues knew where he stood, in this life and the next one.

I spend a lot of time on this site and others talking about sea (and sometimes air) voyages. And I’ve spent most of my career (and all the academic part of same) in the applied sciences. But when I take my last voyage into eternity, I want to do it in the same hope of Newton and Pascal–and Saint-Venant and Euler for that matter–namely that which comes from following Jesus Christ out of the grave and into eternal life.